Abstract
The formation of ribosomal DNA (rDNA) not associated with the nucleolar organizer (NO) regions was studied in polytene cells of Drosophila melanogaster mutants, mal12 and bb2rl, heterozygous for deficiencies in the NO. In the mutant X chromosome mal12 a smaller part of the NO is deleted than in bb2rl but it comprises the compensatory response (cr) locus, controlling the compensatory synthesis of rDNA. We found that in the polytene cells of all tissues of mutant X/Xmal12 investigated (larval salivary gland, fat body and midgut; adult fly midgut) the number of nucleoli was increased compared with that in the corresponding cells of X/Xbb2rl and X/X (wild type). Using in situ hybridization we showed that in the salivary gland cells of the X/Xmal12 larvae chromosomal sites containing type I insertion sequences and scattered throughout the genome were more frequent than in the wild type and in X/Xbb2rl mutant cells.
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